Solar cells and other photoelectric conversion elements hold great promise as sources of clean energy, and p-n junction-type silicon solar cells are already in practical use. However, highly pure raw materials are required for manufacturing silicon solar cells, and vacuum processes and high-temperature processes at temperatures around 1000° C. are also required during preparation of silicon solar cells. Thus, reducing the manufacturing costs of photoelectric conversion elements has been a major issue.
Under these circumstances, recent attention has focused on wet solar cells, in which charge separation is accomplished by means of the potential gradient at a solid-liquid interface. The need for highly pure raw materials and high-energy processes is less with a wet solar cell than with a silicon solar cell.
In recent years in particular, there has been extensive research into so-called dye-sensitized solar cells, which comprise a semiconductor electrode supporting a sensitizing dye that absorbs light. In a dye-sensitized solar cell, the sensitizing dye absorbs visible light at wavelengths longer than the band gap of the semiconductor electrode, and the resulting photoexcited electrons are injected into the semiconductor electrode, improving the photoelectric conversion efficiency.
In a conventional dye-sensitized solar cell, only a single layer of sensitizing dye supported on the surface of the semiconductor electrode injects electrons into the semiconductor electrode. However, as described in Japanese Patent No. 2664194, Gratzel et al proposed that the area of interface between a photosensitizing dye and a titanium oxide electrode could be greatly increased by using a porous titanium oxide electrode as the semiconductor electrode, and supporting the photosensitizing dye on this titanium oxide electrode. The porous titanium oxide electrode is prepared by the sol-gel method. This titanium oxide electrode has a porosity of about 50%, and a porous structure with an extremely large actual surface area. If the titanium oxide electrode is 8 μm thick for example, the roughness factor of the electrode (ratio of actual surface area to projected area) is about 720. The amount of dye supported on this titanium oxide electrode reaches 1.2×10−7 mol/cm2 according to geometric calculation, and in fact about 98% of incident light is absorbed at the maximum absorption wavelength.
The primary features of this new kind of dye-sensitized solar cell (also called a Gratzel cell) are the use of a porous titanium oxide electrode to greatly increase the supported amount of sensitizing dye, and the development of a sensitizing dye providing high absorption efficiency of solar light and extremely rapid rates of electron injection into the semiconductor.
Gratzel et al developed a bis(bipyridyl) Ru(II) complex as a sensitizing dye for a dye-sensitized solar cell. This Ru complex has the structure cis-X2 bis(2,2′-bipyridyl-4,4′-dicarboxylate) Ru(II), wherein X is Cl—, CN— or SCN—. The fluorescent light absorption, visible light absorption, electrochemical behavior and photoredox behavior of these sensitizing dyes have been studied systematically. Of these sensitizing dyes, cis-(diisocyanate)-bis(2,2′-bipyridyl-4,4′-dicarboxylate) Ru(II) has been shown to have far superior performance as a sensitizing dye for dye-sensitized solar cells.
Absorption of visible light by this sensitizing dye is by means of charge transfer transition from a metal to a ligand. The carboxyl groups of ligands in the photosensitizing dye coordinate directly to Ti ions on the surface of the titanium oxide electrode, resulting in close electronic contact between the photosensitizing dye and the titanium oxide electrode. It is said that as a result of this electronic contact, injection of electrons from the photosensitizing dye into the conduction band of titanium oxide occurs at extremely rapid speeds (1 picosecond or less), and recapture by the photosensitizing dye of electrons injected into the conduction band of titanium oxide occurs at speeds on the order of microseconds. This speed difference creates directionality of movement of the photoexcited electrons, which is why charge separation is so efficient. This is the essential feature of a Grätzel cell, distinguishing it from p-n junction-type solar cells in which charge separation is achieved by means of the potential gradient at the p-n junction surface.
In a photoelectric conversion element of the dye-sensitized type, a Ru complex, merocyanine or the like is used as the photosensitizing dye (Patent Document 1).    [Patent Document 1] Japanese Patent No. 4080288